docs: def _get_mainchain(df, invert):
          """Return only main chain atom entries from a DataFrame"""
          if invert:
              mc = df[(df['atom_name'] != 'C') &
                      (df['atom_name'] != 'O') &
                      (df['atom_name'] != 'N') &
                      (df['atom_name'] != 'CA')]
          else:
              mc = df[(df['atom_name'] == 'C') |
                      (df['atom_name'] == 'O') |
                      (df['atom_name'] == 'N') |
                      (df['atom_name'] == 'CA')]
          return mc